This invention relates to an improved method and apparatus for the storage of digital data on a magnetic surface. The invention relates more particularly to an improved method and apparatus for encoding the stored digital data in order to enhance its storage and recovery.
Various high capacity magnetic storage techniques are known for storing data occurring in the form of digital electrical signals. Among these techniques, and with which this invention is concerned, is the storage of digital data on a magnetic surface such as is provided by a magnetic tape, disc or drum. In these arrangements, an elongated track of magnetic material is transported past a station at which location a residual magnetic pattern corresponding to a pattern of bits of binary information being stored is formed on the track.
An important consideration in the storage of data in this manner is the packing density of the bits of stored data. The storage capacity, physical size and cost of a system is dependent to a large extent upon the density per unit length of track with which binary bits are stored on the magnetic medium. A single bit of binary information is stored in a cell which comprises a storage area extending along the length of the track. A longitudinal array of the cells is formed on the track and bits are stored in cells as transitions of residual flux. While providing a relatively short separation time (T.sub.min) between transitions reduces cell size and increases the packing density, other considerations impose restrictions on the bit density. These considerations include the capability of a recording/readout means to reliably form the transitions during storage and to distinguish between the relatively closely spaced transitions during readout.
Various techniques for encoding the binary information being stored have been employed in order to enhance detection and recovery of the data and thus to increase the bit packing density. In a phase modulation [PM] technique, a positive flux transition is centered in a data cell when a first binary character such as a binary 1 is being stored and, alternatively, a negative flux transition is centered in the data cell when storing a second binary character such as a binary 0. A frequency modulation [FM] technique provides for two flux transitions in a cell when storing a binary character 1 and a single flux transition for storing a binary character 0. In a modification of this frequency modulation technique, [MFM], one binary character is represented by a flux transition in the center of a cell while the second binary character is stored as a flux transition at a cell boundary. A further modification is provided in the latter [MFM] coding technique by inhibiting a transition when the first and second binary characters occur in predetermined sequences in immediately adjacent cells. More particularly, in one modified frequency modulation technique, also known as the "look back" technique, a transition at a cell boundary representing the second binary character is inhibited when it succeeds the first binary character in an adjacent cell. In another modified frequency modulation technique known as the "look ahead" technique, a transition representing the second binary character is inhibited when it preceeds the first character in an adjacent cell.
The bit packing density provided by these encoding techniques as well as prior techniques is affected in large part by the interval of minimum separation (T.sub.min) existing between flux transitions. Maximum separation on the other hand is significant in determining the bandpass of the system, the self clocking characteristic of the system and the susceptibility of the system to speed variations occurring in a traveling magnetic track. The recording of binary data on a magnetic track and particularly on magnetic tape is characterized by a severely limited bandpass. It is therefore desirable that the encoding technique employed to enhance packing density provide efficient bandwidth usage in order to be compatible with a restricted recording bandpass. The bandpass for binary data encoded as flux transitions is a function of the ratio of maximum transition separation to minimum transition separation: BW is proportional to T.sub.max /T.sub.min. The provision of a relatively short T.sub.min increases packing density. A relatively long T.sub.max however reduces inherent self clocking capabitities of the storage system and increases the susceptibility of the system to variations in the speed of a traveling magnetic track.
Although the encoding techniques referred to hereinbefore enhance detection and recovery of data thereby providing for improved packing density, it is desirable to provide binary data encoding of relatively reduced maximum transition time T.sub.max.
Accordingly, it is an object of this invention to provide an improved method and apparatus for storing digital data on a magnetic surface.
Another object of the invention is to provide an improved method and apparatus for encoding binary data for storage on a magnetic surface.
A further object of the invention is to provide an encoding method and apparatus having a relatively reduced maximum transition time T.sub.max.
Still another object of the invention is to provide an improved encoding method and apparatus useful in both "look ahead" and "look back" recording techniques.